1. you don't need that bulky PC PSU. 2. no need to manually switch on/off. 3. the extra power for the 3rd Titan X is just like the on board pcie power, no need to take care of it when power on or off since you have a signal line connected with the system.

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Here is the photo of the nice small switch to determine whether the extra power from the Xbox 360 175W PSU is needed. But don't be fooled. You don't have to switch manually to get the extra power. The switch is for with or without the 5v signal, meaning I would have switch it off if only two cards are on board. The 5v signal line for the switch is from the optic drive connection. I still have 3 SATA port to use (SATA3 SSDs all have to be naked, no choice). The extra power runs so smooth. You won't feel it at all (there was no fan noise on that). It's just like the Mac Pro 5.1 come with 3 pair of 12V aux power for GPUs.
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I leave the pins jumped. That way you can omit steps 2 and 5. You can also get a pre made jumper from Amazon. I have one but for whatever reason stuck with the simple paper clip trick.

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Thanks for your reply. Thats helpful to know you always leave the PS_ON and ground connected and just switch it on and off with the PSU on/off switch. I thought maybe doing that might be bad for the PSU, but if other people think its fine then maybe I should just do that.

The rm550x power supply I have is modular so it would be great if I could use it without the ATX cable. I was wondering if I could get a 18 pin ATX connector and connect the PS_ON and ground together like the below pin out guide I found. Does anyone think this would be a bad idea?

1. you don't need that bulky PC PSU. 2. no need to manually switch on/off. 3. the extra power for the 3rd Titan X is just like the on board pcie power, no need to take care of it when power on or off since you have a signal line connected with the system.

------------
Here is the photo of the nice small switch to determine whether the extra power from the Xbox 360 175W PSU is needed. But don't be fooled. You don't have to switch manually to get the extra power. The switch is for with or without the 5v signal, meaning I would have switch it off if only two cards are on board. The 5v signal line for the switch is from the optic drive connection. I still have 3 SATA port to use (SATA3 SSDs all have to be naked, no choice). The extra power runs so smooth. You won't feel it at all (there was no fan noise on that). It's just like the Mac Pro 5.1 come with 3 pair of 12V aux power for GPUs.
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Thanks for your reply Prince134. Yeah I have already seen your great mega mega build. I didn't want to solder onto the Mac Pro PSU, so the external PSU seemed the best option for me.

Oh and thanks for your tips about the evga power link. I bought one of these to help better share the 6pin + 8pin of the GTX 980 ti from the two mini 6pins of the motherboard.

Your three GPU setup is impressive - I was wondering if it was throttling with heat without any gaps between the cards. I am using this system for rendering in Octane, and it would be left on overnight, so as much as I love the idea of try to get another 980ti card into my Mac, I worry that they would overheat. (at the moment I have the two of them with a gap between). Maybe I should just do more benchmark tests of the speeds of the cards next to each other, and a gap between and see how much that changes the render speed.

The blower design has taken into consideration of that. All double wide cards sitting 2way or 3way sli has the same gap. The pcie fan will kick in when needed to blow in outside cool air to GPUs. In fact, octane rendering consume less total wattage than gaming since most of the time CPU is idle. Of course Mac Pro is not server, so the fan noise is lower. I understand your concern. Whoever wants to burn their machine with fur-mark all night? I never will.

If you have two cards, you want the gap, then put slot2, 4. or slot1, 4.

The blower design has taken into consideration of that. All double wide cards sitting 2way or 3way sli has the same gap. The pcie fan will kick in when needed to blow in outside cool air to GPUs. In fact, octane rendering consume less total wattage than gaming since most of the time CPU is idle. Of course Mac Pro is not server, so the fan noise is lower. I understand your concern. Whoever wants to burn their machine with fur-mark all night? I never will.

If you have two cards, you want the gap, then put slot2, 4. or slot1, 4.

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Thanks for your thoughts about the triple stacked cards. That is really interesting to hear that is how those graphics cards are designed to be used together in a SLI gaming setup.

My cards are the eVGA GTX 980 ti SC+, and it is my understanding that the cooling of these blows the air around inside the case, rather than throwing it out the back like a reference design blower card...

Do you think it would be a bad idea to try and fit three eVGA GTX 980 ti SC+ cards in my MacPro? or do you think that is only ok if all the cards have reference design fans?

(also I wonder if it is a good idea to remove the backplates of my cards? Are back plates for gamers who want the inside of their machines to look cool, rather than helping cooling? I'm thinking getting the most space possible around the cards is better for cooling.

I think you have the same modular power supply as me - Corsair RM550x, it would be great if there was a one of those adaptors that fitted straight onto the 10+18pins of the power supply, rather than the 24 pin tax cable that plugs into the power supply. Don't think this exists though.

Thanks for your thoughts about the triple stacked cards. That is really interesting to hear that is how those graphics cards are designed to be used together in a SLI gaming setup.

My cards are the eVGA GTX 980 ti SC+, and it is my understanding that the cooling of these blows the air around inside the case, rather than throwing it out the back like a reference design blower card...View attachment 700966

Do you think it would be a bad idea to try and fit three eVGA GTX 980 ti SC+ cards in my MacPro? or do you think that is only ok if all the cards have reference design fans?

(also I wonder if it is a good idea to remove the backplates of my cards? Are back plates for gamers who want the inside of their machines to look cool, rather than helping cooling? I'm thinking getting the most space possible around the cards is better for cooling.

Thanks!

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You can use this kind of cooler for the slot 4. But remember you have to choose one that without the DVI socket to put in slot 4. I don't think any GTX 980 Ti without DVI socket. Only 1080 TI and Titan xP that I know of. The other 2 cards has to be reference design, other wise the fans shown aboove just get stock with each other.

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